I did the test and saw 3 F's, then I looked again and saw all 6.
First I was thinking there was something in a script.
refreshed the page - still 6.
Asked Viv (sitting next to me) to look and count the F's : 3.
Asked here to say it up loud and count how much F's she heard - she still said 3.
Then I said of-of-of and she "saw" the f's.
Strange.